Arenaviruses such as Lassa fever virus (LASV) are known to cause deadly hemorrhagic fever diseases in human with little preventive or prophylactic measures. A hallmark of arenavirus-induced hemorrhagic fevers is the general immune suppression in severe and lethally infected patients. The current lack of knowledge on how arenaviral infections lead to host immune suppression is a major barrier to understanding the virulence nature and disease pathogenesis of these human pathogens and has hindered the development of effective vaccine and therapeutic intervention. Arenavirus nucleoprotein (NP) has been shown to suppress the interferon (IFN) induction, the mechanism of which, however, remains elusive. We have recently reported the first crystal structure of the LASV NP protein (Qi, et al, Dec 2010, Nature). An unexpected finding from our structural and functional analyses is that the C domain of NP folds into a functional 3'-5' exoribonuclease (RNase) that degrades various types of RNA substrates and is essential for the NP's ability to suppress the IFN induction. As IFNs are induced by the cellular pattern recognition receptors (PRRs) that recognize viral pathogen-associated molecular patterns (PAMP) RNAs, we hypothesize that arenaviral NP proteins evade host innate immunity by specifically degrading these PAMP RNA ligands via its exoribonuclease activity. LASV NP is the first virally encoded RNase that uses exoribonuclease activity to block the IFN induction, which represents a novel mechanism of viral immune evasion. The goal of this proposal therefore is to provide mechanistic insights into this novel approach of viral immune evasion by arenavirus NP exoribonuclease activity. Specifically, we will define the structural basis of LASV NP in mediating IFN suppression and exoribonuclease activities (Aim #1) and determine its RNA substrate specificity (Aim #2). Finally, we will complement and extend biochemical and cell-based studies outlined in Aims #1 and #2 to determine the role of NP RNase in mediating host immune suppression and pathogenesis using a disease-relevant small animal model (Aim #3). The novelties of the study are defined not only by the unique concept of how virally encoded RNase mediates the IFN suppression but also by a combination of unique experimental systems that integrate protein structural biochemistry, molecular and cellular biology, infectious virus, and a pertinent animal model for understanding human hemorrhagic fever diseases. The proposed study is highly significant as it will reveal for the first time a novel molecular mechanism of immune evasion mediated by arenaviral NP protein that may offer opportunities for development of effective therapeutics and/or vaccines against lethal arenaviral hemorrhagic fever infections in humans. As LASV NP is the first identified viral RNase with IFN suppression function, this study has a broaden significance in understanding the virus-host interactions, in particular, on how viral pathogens employ various ingenious strategies to evade host immunity.